Fretting wear and cracking in sintered metal matrix composites.

Abstract

A methodology, involving fretting tests, to develop wear and crack resistant materials for tribological applications for automotive valve train parts (e.g. cams, tappets) has been recently reported for high speed steels. Modifications to one of these sintered steels, M3 Class 2, were effected by additions, singly and in combination, of 5 wt.% of wear resistant titanium carbide and of solid lubricant manganese sulphide. In our fretting tests alternate displacements were imposed between the test material (plane) and a chromium steel or alumina ball. Running conditions fretting and material response fretting maps were constructed for the four materials. Two types of fretting damage were detected and analysed: cracking or particle detachment and wear through the tribologicaly transformed structure (TTS). Crack initiation, associated with porosity and interfaces, was detected when the maximum tensile stress in the contact reached 1.2 GPa. Cracking analyses were also carried out using static and fatigue mechanical tests and replica scanning electron microscopy. Crack growth and propagation were influenced by details of the microstructure, e.g. TiC was observed to arrest crack growth, whereas MnS made it easier. Wear analysis included the determination after each test of the wear volume, which could be related to the coefficient of friction and the cumulative dissipation energy during the fretting test.